1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic system and, in particular, to a technique for controlling a light amount of a light beam emitted from a light source.
2. Description of the Related Art
The image forming apparatus of the electrophotographic system is provided with a light source (for example, an LED, a semiconductor laser) for exposing a photosensitive member. The image forming apparatus controls a driving current supplied to the light source so that an output image is formed with a desired density.
Japanese Laid-Open Patent Publication (Kokai) No. 2000-351232 (JP 2000-351232A) discloses a technique that stores a driving-current/light-amount characteristic (a light-amount/current characteristic curve) to a nonvolatile memory for every LED and controls a light amount of a light beam emitted from each LED on the basis of the driving-current/light-amount characteristic in order to correct variation among characteristics of a plurality of LEDs and characteristics of light-amount control units.
However, the technique of the above-mentioned publication cannot cope with a change of the driving-current/light-amount characteristic due to a temperature change around the light source, a temperature change of the light source due to heat generation of the light source itself, or aging degradation of the light source that is caused by repeating emissions.
FIG. 21 is a graph showing examples of driving-current/light-amount characteristics of a semiconductor laser.
In FIG. 21, a solid line indicates the driving-current/light-amount characteristic in a case where the ambient temperature of the light source is 25° C., and a broken line indicates the driving-current/light-amount characteristic in a case where the ambient temperature of the light source is 50° C. As shown in FIG. 21, the light amount of the light beam emitted from the light source is 1.00 mW in a case where the driving current supplied to the light source is 1.84 mA at the ambient temperature of 25° C. On the other hand, the light amount of the light beam emitted from the light source is 0.86 mW in a case where the driving current supplied to the light source is 1.84 mA at the ambient temperature of 50° C. Thus, the driving-current/light-amount characteristic varies due to the change of the ambient temperature of the light source, etc.